Solid core golf balls are well known in the art. Typically, the core is made from polybutadiene rubber material that provides the primary source of resiliency for the golf ball. A known drawback of polybutadiene cores cross-linked with peroxide and/or zinc diacrylate is the adverse effect of the absorption of water vapor, which can degrade the performance of the core material, in particular resilience.
Prolonged exposure to water vapor (i.e., at high humidity) and elevated temperature may be sufficient to allow the water vapor to invade polybutadiene cores. For example, at 38° C. and 90% humidity over a sixty-day period, a sufficient amount of moisture permeates the core to reduce the initial velocity of the golf ball by about 1.8–4.0 ft/s, sometimes greater. The change in compression may be reduced by 5% or more. Absorbed water vapor can also reduce golf ball coefficient of restitution (“COR”). When a golf ball is subjected to prolonged storage and/or use under humid conditions (i.e., 25–35% relative humidity), or conditions exhibiting a combination of high temperature and humidity, the COR of the golf ball decreases over time due to water vapor absorption.
Therefore, cores of this nature must be covered quickly to maintain optimum ball properties. A cover that protects the core from the elements and repeated impacts from golf clubs is typically made from ionomer resins, balata, or urethane, among other materials. Ionomer covers, particularly hard ionomers, offer some protection against the absorption of water vapor. However, it is more difficult to control or impart spin to balls with these types of covers. Urethane covers can provide better ball control but offer less resistance to water vapor than do ionomer covers. Several prior patents have addressed the water vapor absorption problem. These include U.S. Pat. Nos. 5,820,488; 5,885,172; 6,132,324; 6,232,389; and 6,287,216, which are incorporated by reference herein, in their entirety. Manufacturers have typically resorted to the incorporation of a WVB material in an existing layer or a designated WVB layer to overcome the water vapor absorption problem in golf balls, escalating manufacturing cost and construction complexity. It is therefore desirable to provide cost-effective material compositions that have general characteristics suitable for golf ball constructions with concomitant WVB properties.
One particular class of polymers suitable for golf ball compositions of the present invention is fluoropolymers. Fluoropolymers are well known for their excellent resistance to outdoor weathering and ultraviolet irradiation, high degree of physical toughness, chemical inertness, water and gas impermeability, low moisture absorption, good abrasion resistance, non-tackiness, resistance to soiling, as well as a significant retention of these properties at both low and elevated temperatures. This rare combination is due to the strength of the C—F bond, one of the most stable bonds known. Other desirable properties of the fluoropolymers include high elongation, low coefficient of friction, low wear rate, good resistance to swell in solvents, low refractive index, and low dielectric constant. These desirable properties allow the fluoropolymers to be widely used as self-supporting layers or outer coatings on various substrates. Disclosures of fluoropolymer applications in golf ball compositions, however, are limited, and include U.S. Pat. Nos. 5,962,140; 6,133,389; and 6,217,464, which are incorporated by reference herein, in their entirety.
The major hindrance to the use of fluoropolymers for golf ball layers is their poor adhesion (and, hence, the reason why they excel for use in non-stick cookware), especially when formed as a very rigid layer. Rigid layers are sometimes preferred when layer thickness is very thin, such that the properties of the layer are still an integral part of golf ball performance. In general, the more rigid a fluoropolymer layer is, the more difficult it is to manufacture in a manner that resists layer breakage and cracking due to poor adhesion.
Because most polymers containing fluorine atoms have difficult adhesive properties, there remains a need for further development of golf ball compositions comprising fluoropolymers, particularly non-ionic hydrophobic stiff (“NIHS”) fluoropolymers, in multi-layer golf ball constructions, preferably inner cover layers, in combination with surface chemistry modifiers of polybutadiene core chemistry.